On internal combustion engines, particularly car engines, the exhaust gas produced by combustion of the fuel in the engine contains pollutant or toxic gases and vapours due to imperfect combustion of the hydrocarbons and to the additives used in the fuel to improve the heat cycle, and is fed into and discharged from an exhaust pipe normally fitted at the end with a silencer for absorbing the peak sound waves and deadening the overall noise produced by the exhaust gas.
As is known, car engine fuels--petrol, L.P.G. or methane in spark-ignition combustion engines, and gas oil in diesel engines--comprise hydrocarbons of the general chemical formula CnHm, i.e. having more or less large carbon and hydrogen molecules of various atom arrangements. The atmospheric air drawn into the engine for combustion is a mixture of nitrogen, oxygen and water vapour--the latter varying according to the degree of humidity--and also contains other gases and vapours depending, obviously, on the degree of pollution of the atmosphere in which the engine is operated.
If the oxidation process were perfect, the chemical reaction would be: EQU CnHm+(2n+m/2)O.fwdarw.nCO.sub.2 +m/2H.sub.2 O
and only carbon dioxide and water vapour molecules would be produced. In actual practice, however, internal combustion engines also emit unburnt hydrocarbon vapours, other noxious gases such as nitric oxide, and toxic gases such as carbon monoxide.
The composition and percentage of toxic or noxious exhaust gas components vary considerably from one engine to another, and the percentage is particularly high in traffic jams or particular atmospheric conditions in which the gas settles and cannot be dispersed readily into the atmosphere. In certain urban areas, high-molecular-weight gas, such as carbon dioxide and hydrocarbon vapours, forms a layer which concentrates infrared rays and results in the so-called greenhouse effect responsible for sudden changes in weather.
Moreover, to improve compression in the cylinders and prevent spontaneous ignition of the mixture, spark-ignition combustion engines employ petrol containing antiknock additives, in particular tetraethyllead in leaded, so-called red, petrol, which is notoriously toxic. To eliminate the noxious effect of tetraethyllead, unleaded or so-called green petrol has recently been developed, in which the antiknock additive comprises an aromatic hydrocarbon MTBE (methyl-ter-butyl-ether) or benzene. Like most unburnt hydrocarbons and polymers, however, unleaded petrol is cancerogenic and should therefore only be used in engines equipped with a catalyzed muffler featuring a lambda probe and in which the unburnt hydrocarbons from the cylinders are burnt.
In diesel engines, in which fuel combustion occurs spontaneously by injecting the fuel into the combustion chamber at the compression stroke, combustion is frequently incomplete for various reasons--fuel supply, injector settings, engine speed and load, warm-up operation of the engine--so that the exhaust gas is more or less dark in colour, due to waste and/or various unburnt hydrocarbons forming the so-called "particulate" of the gas.
On modern spark-ignition combustion engines equipped with catalyzed mufflers, and diesel engines equipped with electronic-control injectors, toxic emissions have been greatly reduced, but absolutely no reduction has been achieved in the emission of pollutant substances, such as CO.sub.2.
From document U.S. Pat. No. 3,983,021, it is known a nitrogen oxide decomposition device for the exhaust gas of internal combustion engines, wherein the gas is conveyed to a dielectric tubular pipe having on its outer surface a conductive covering and containing an inner electrode in form of a cylindrical bar. The electrode and the covering are connected to a high voltage generator to provide a plasma discharge into the pipe, in which high surface area packing material is disposed. The nitrogen oxide are decomposed by the the ionizing action of the discharge and by the absorption action of the material.
From document U.S. Pat. No. 3,979,193, it is known a corona discharge apparatus for removing pollution of gas of internal combustion engines into a corona-producing chamber. The longitudinal wall of this chamber is formed of an electrically conductive material and acts as a reactor for a corona emission produced by a coaxial coupling rod, which is provided with a set of discs, each having serrated peripheral edge of small surface area. The gas to be introduced into the chamber are mixed with air to provide an amount of oxygen within the exhaust for converting carbon monoxide to carbon dioxide.